R: DISH BEAM WIDTH

Hi Bill WA1DMV,
A 12' dish (3.65 meters) in diameter at 2400 MHz has a -3 dB beamwidth of
2.2 degrees in the H plane and 2.5 degrees in the E plane
The math is too long.
Assuming an efficiency of 50% the gain is 31.5 dBi
A simple but less accurate formula to calculate the -3dB beamwidth
is to calculate the ratio lamda/D using the same units where lamda is the
wavelengt and D is the diameter of the dish and than converting the result
of this ratio from radians to degrees.
In your case:
lambda= 12.5 cm
D= 365 cm
12,5/365= 0.0342 radians ; 0.0342 x 57.29= 1.96 degrees wich is very close
to the above H and E plane angles to be calculate with a more complicated
math bat for our amateur use the result is OK
The F/D ratio is important only to designe the type and beamwidth of the
feed to be used in order to illuminate the edge of the dish at -10 dB and
this requires simple math that you can find in any antenna book.
73" de i8CVS Domenico
----- Original Message -----
From: <walnuthil@juno.com>
To: <AMSAT-BB@AMSAT.Org>
Sent: Saturday, March 01, 2003 6:24 AM
Subject: [amsat-bb] DISH BEAM WIDTH
> Hello All,
> I recently acquired a TVRO 12 foot dish, with the mesh
> reflector. I have been trying to calculate the 3db beam width
> in degrees, the books I have are not clear on the formula.
> Does anyone have the correct math formula ?
> Putting a 2 by across the dish and measuring down the
> center is 22 inches. Yields a f/D = 4.0 if the formula is right.
> I plan on using it for receive on AO-40, I have some tall pine
> trees to the south that reduce the down link signal.
> Hopefully this dish will help.
> Thanks in advance
> Bill
> WA1DMV@amsat.org
> #20942
>
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